1. Field of the Invention
The invention relates to a light-metal low friction piston in a cylinder of an internal combustion engine.
2. The Prior Art
Designs for light-weight and low friction pistons are known in the prior art, e.g., from DE 3,430,258 A1, DE 3,446,121 A1 and EP 0,171,568 A3. With these pistons, as a rule, their noise level is unsatisfactory, due to the extremely short body lengths of the pistons disclosed in these patents and in EP 0,171,568 A3. The piston cannot be guided accurately in such a way that a piston impact, together with a striking of the piston head against the cylinder track, can be reliably avoided under all operating conditions. Striking or impact of the piston head against the cylinder track leads in turn to undesired high levels of piston movement noises. This is especially for engine operation in the start-up and partial load state in which piston body play, because of the low body temperatures occurring at these operating states, has not yet reached its lower value during engine load operation. The lower value during load operation results from the heat-dependent expansion of the body material, whereby expansion in the pressure-counter-pressure direction of the piston can be further reduced by means of expansion-regulating strips.
From DE-AS 1,078,387, a method is already known in the prior art for creating, aside from on the piston body, also on the straps between the piston ring grooves located in the piston head area, a tight play vis-a-vis the cylinder track surface. Those pistons are primarily designed for port-controlled, two-stroke engines, in which a tight operating tolerance at the ring groove straps is required for exact control of the gas scavenging port in the cylinder track. With the relatively long pistons described exclusively in that publication, a tight operating tolerance in the piston head area results in the fact that the operating tolerance cannot be simultaneously extremely narrow in the body area as well. For, practically speaking, such precise fabrication is not possible over a great length of the piston, such that a continuously extremely tight running play over the entire height of a long piston would statically overdefine the latter within the engine cylinder. This would result in the piston's being unable to move. A partial asymmetrical, radial expansion of the piston over its height must also be taken into consideration in such cases, which expansion can also cause a locking within the engine cylinder for long pistons. The former piston known in the prior art, even with considerable modifications of its indicated dimensions, is therefore in no way suited to solve the problem which is solved by the present invention, namely, of creating a short, light weight and nevertheless noiselessly running piston.
The ring inserts in the piston head according to the invention differ from similar inserts described in EP-A 0,210,649 particularly in that, contrary to those described in the prior known state of the art, they are not arranged in the top land area above the annular grooves. Instead they are located radially in the area within the annular grooves. In that way they can directly bear upon the area of the ring straps at which, through restriction of the heat expansion, an operating tolerance as tight as possible would exist in the cold state and would be maintained under the hot operating conditions.